Field of the Invention
This invention relates to a dry etching method. More particularly, it relates to a dry etching method with which it is possible, in forming a fine resist pattern, to suppress micro-loading effects, achieve a practically useful etching rate and improve anisotropy.
Recently, with refining of a design rule of semiconductor devices, a demand for higher resolution and shape anisotropy is increasing acutely. As a technique of producing such a resist pattern, there is known a multi-layer resist process in which at least two resist layers, namely an upper resist layer adapted to be uniformly exposed to light across its thickness to assure high resolution, and a lower resist layer having superior properties in covering step differences on a base layer, are used in combination. Recently, an exposure device known as an eximer laser stepper capable of coping with a 0.35 .mu.m rule has been developed and a multi-layer process is becoming an indispensable technique in the field of photolithography employing such light exposure device.
However, it is difficult to carry out the multi-layer resist process at a higher rate and with good controllability mainly because an oxygen gas is used in general for etching the multi-layer resist process. The oxygen gas tends to produce oxygen radicals O* in a gaseous phase which are responsible for lowered anisotropy and manifestation of the micro-loading effects. For coping with these inconveniences, extreme physical conditions, such as high bias voltages or low gas pressures, may be used to increase an energy of an incident ion energy or to elongate the mean free path of etchants. However, these measures tend to lower the etching rate drastically or to increase etching damages.
Various investigations into the technique of possibly overcoming these inconveniences are proceeding briskly. The present assignee has also proposed (a) a technique of using N.sub.2 as an etching gas, (b) a technique of using NH.sub.3 as an etching gas and (c) a technique of using an etching gas containing both O.sub.2 and Cl-based gas.
Of these, the technique (a) has been proposed by the present assignee in the JP Patent Publication KOKAI No. 1-215024 (1989). It is possible with this technique to perform anisotropic etching under a lower bias voltage than that used in case of using O.sub.2.
The technique (b) of using NH.sub.3 as an etching gas has been proposed by the present assignee in our copending JP Patent Publication KOKAI NO. 1-280316 (1989). It is possible with this technique to realize a higher etching rate than that when using N.sub.2. A technique of controlling the temperature of the etched substrate to not higher than 50.degree. C. to allow a reaction product to be deposited partially to lower the micro-loading effects has also been reported by a group of researchers, of which the present inventors are members, in Extended Abstracts in the 51st Autumn Meeting, 1990, in the Japan Society of Applied Physics, second volume, page 459, title number 26a-ZF-2.
The technique (c) of using an etching gas containing both O.sub.2 and Cl-bases gas has been reported by a group of researchers, of which the present inventors are members, in Extended Abstracts of the 22nd Conference on Solid-State Devices and Materials, pages 203 to 206 (1990). With this technique, a sidewall protection film is utilized to achieve anisotropic etching.
It is also contemplated to effect etching by cooling the base plate to be etched to a temperature not higher than 0.degree. C. This is a so-called low temperature etching which is recently attracting attention in the field of dry etching. With the low temperature etching, as reported on pages 42 to 49 of Extracts of Dry Process Etching Symposium of 1988, the base plate to be etched is maintained at a lower temperature to halt a radical reaction on the sidewall to prevent shape defects, such as side etching, as the etching rate along the depth is maintained by an ion assist effect. As an example of application to resist etching, there is reported in Extended Abstracts, in the 36th Spring Meeting of the Japan Society of Applied Physics and Related Societies, page 574, lecture number 1p-L-15, an example of etching a lower resist layer of a three-layered resist pattern, with the use of an O.sub.2 gas, as the wafer is cooled to a temperature not higher than -100.degree. C.
However, the above depicted conventional measures suffer from several drawbacks. First, with the use of NH.sub.3 as an etching gas, anisotropic etching under a lower bias voltage may be achieved more promptly than with the use of N.sub.2 gas, however, with an electron cyclotron resonance (ECR) system or a magnetron system of the high density plasma generating type, the operation of hydrogen radicals H* yielded upon dissociation of NH.sub.3 becomes manifest to produce undercuts. With the use of the etching gas containing O.sub.2 and Cl-based gas, amorphous silicon etc. is etched by the operation of the Cl-based gas, so that the material of the intermediate layer in the three-layer resist process is necessarily limited to the silicon oxide type material. In case of the low temperature etching, the temperature need to be lower than -100.degree. C. to allow for deposition of a reaction product contributing to anisotropy. Hence, existing systems cannot be used for low temperature etching in consideration of the necessity for providing a cooling system or measures against dewing. In addition, the throughput of the system may be lowered in consideration of the time necessary for cooling or raising the temperature to room temperature.